A:could buy
B:should buy
C:must have bought
D:could have bough
2050
已知OA→=1,OB→=3,∠AOB=5π6点C在∠AOB外且OB→·OC→=0,设实数m,n满足OC→=mOA→+nOB→则mn等于( )
人们为了防止鸟类吃草籽,把一片草地用网罩起来,结果却使食草昆虫大量繁殖,草的叶子几乎被吃光,产生这种现象的原因是( )